Certain types of electrical equipment, for example a digital signal processing system, often have components mounted on printed circuit cards, which are inserted into a framework. A back piece of the framework consists of a connection device (also called a back plane or a mother card). This is a circuit card which has edge contacts for each printed circuit card and an electrically conducting pattern consisting of a number of, usually parallel, conductors which connect the edge contacts of each card to each other. The conductors consist of so-called bus conductors (or buses) and are employed for transmitting supply voltages to the circuit cards and for transmitting signals between the circuit cards. The edge contacts of the circuit cards achieve electrical contact between a circuit card and the respective conductors of the connection device when the card is plugged into position in the framework. At the front edge of the circuit cards (i.e. the edge facing away from the connection device) further contacts may be arranged for connecting the cards to external devices, for example an appliance controlled by the equipment, and/or to indicating or operating devices.
The equipment is often divided into functional units, which may each comprise one or more printed circuit cards. One example of a functional unit is an A/D converter. The functional units and the communication between them may be controlled by a programmed unit, for example a microcomputer arranged on a printed circuit card in the framework.
The need for signal transmissions within a functional unit is usually considerably greater than the need for signal transmissions between separate functional units. There is therefore a need to use certain of the conductors of the connection device (i.e. "local buses") for transmitting signals between the printed circuit cards which make up a multi-card functional unit. The conductors used for this purpose must not then extend beyond the outermost circuit cards which make up that functional unit. It will thus often be necessary to employ a special connection device for each specific piece of equipment, the local buses being provided with interruptions at the correct positions to locate functional units. This can become complicated and expensive, and furthermore such a specially adapted piece of equipment will be difficult to change or extend, since each functional unit has a precise location specified from the beginning in the design of the connection device.
Instead of the above-described arrangement, it would be possible to divide the connection device into modules, with one module for each functional unit. However, such a solution would involve problems in accommodating bulky connection members for the bridging of supply voltages as well as providing communication for control signals between the modules. Further, it is not practicable to manufacture modules for all conceivable numbers of printed circuit cards per functional unit, and the space within the framework would therefore be poorly utilized.